Several options for improving energy recovery, even in a single-pressureteam system, are illustrated in Figure Make-up water or condensate can be heated in the HRSG itself Figure This reduces the amount of steam required for deaeration, improving the overall efficiency. If sulfuric acid vapor is present in the exhaust gases, the condensate temperature should be no lower than the acid vapor's dew point to prevent condensation of the corrosive vapors on the tube (4). This condensate heater option is generally used in natural-gas fired systems that do not contain acid vapors. Still, the water temperature entering the exchanger should be above the water vapor's dew point to prevent water condensation on the tubes. The second option is to generate low pressure saturated steam or deaeration steam in the HRSG itself using a lowpressure evaporator (Figure 10b). This type of system is recommended if there is a possibility of acid vapor condensation, since the steam saturation temperature can be maintained above the acid's dew point.
However, it is more expensive than the condensate heater option due to higher surface area requirements and the need for a drum, instrumentation, and controls. The exit gas temperature from the HRSG will naturally be higher than the saturation temperature of steam, whereas in the pre vinous option, it could be much lower.
The third option is to preheat the make-up water in a heat exchanger before it enters the deaerator, while simultaneously cooling the feed water before it enters the economizer Figure The economizer requires a larger surface area, but this is an economical option compared to the deaerator.
Condensing heat exchangers have also been used in some projects. Poly tetrafluoroethylene (PTFE; e.g., Teflon) or similar corrosion-resistant material is used as a coating on the tubes to prevent corrosion from acid condensation. In such cases, the make -up water can enter the heater as cool as 60-80°F. Another option for lowering the
exhaust gas temperature is to circulate more water than necessary through the economizer and recirculate the excess to the deaerator in order to reduce the deaeration steam requirements Figure.
Some plants, depending upon the steam system and the quantity and temperature of make-up water required, may use a combination of these methods.
